There have been widely used switching DC-DC converters for converting an input voltage into a required output voltage by switching the input voltage.
Although this type of DC-DC converters has fairly large energy consumption of itself, they can provide power efficiently to a rather heavy load since they have high power conversion efficiency. Unfortunately, when the load is light (less than a threshold level), the power consumption rate of themselves is comparatively large that their overall power conversion efficiency is low.
In order to avoid such loss of conversion efficiency when connected to a light load, a DC-DC converter is used along with a series regulator that consumes a little power, in spite of the fact that the series regulator has low power conversion efficiency. Thus, the DC-DC converter is used when the load is heavy, and the series regulator is used when the load is light, thereby improving the overall power consumption efficiency (see for example, Japanese Patent Applications Laid Open 2002-300769 and 2001-211640).
The prior art power supply unit is adapted to selectively operate either one of the DC-DC converter and the series regulator depending on the condition of the load being heavy of light. However, there is a possibility that the switching is not smoothly executed then. Furthermore, a sudden change in the electric power supplied to a smoothing coil and a smoothing capacitor may cause free oscillations and/or an overshoot in the unit. In the worst case, voltage fluctuations can reset other control circuits.